1. Technical Field
This invention relates generally to semiconductor devices, and more particularly, to a neutron detecting device.
2. Background Art
U.S. Pat. No. 6,075,261 entitled NEUTRON DETECTING SEMICONDUCTOR DEVICE, invented by Hossain et al., assigned to the Assignee of this invention, discloses a neutron detecting device which is formed by providing an array of flash memory cells, with neutron-reactant material over the memory cells. Upon been penetrated by a neutron, the neutron-reacting material emits one or more particles capable of inducing a state change in a memory cell. For example, as disclosed in that patent, the state of the flash memory transistor illustrated and described therein is an on-state or a logical 1 state, associated with a negative charge on the floating gate and an inversion layer beneath the floating gate. In such case, the neutron-reactant material, upon being penetrated by a neutron, emits one or more particles which pass through the inversion layer, sufficiently reducing the charge in the channel region of the transistor to remove the inversion layer and change the state of the memory cell to an off-state or logical 0 state.
The neutron detecting device includes a memory arrangement which includes a plurality of flash memory cells in the form of an array, as described above. Typically, the initial, undisturbed state of each memory cell is set to a logical 1. During a detection cycle, the state of each cell is read to determine whether such state has changed, indicating detection of neutrons in accordance with the above mechanism. The proportion of cells which have changed state compared to the overall number of cells in the array can be used to determine the presence and intensity of a neutron field. In a typical embodiment, the percentage of state changes can range from for example 0.001% to 0.1% of the total number of memory cells in the array. After a chosen time interval during which the reading of the cells takes place as described above, all of the memory cells are reset to logical 1 in preparation for the next detection cycle.
In such a device, a reading of intensity of the neutron field as indicated by the device is dependent on the orientation of the device relative to the path of travel of the neutrons of the neutron field, as will now be described and illustrated with regard to FIGS. 1 and 2.
FIG. 1 illustrates a neutron field 20 which includes a plurality of neutrons 22 flowing in the direction indicated. It will be understood that the neutrons 22 illustrated are a portion of a large neutron field 20, which field 20 extends sidewardly of FIG. 1 and also perpendicular to the plane of FIG. 1. The neutrons 22 are indicated as generally equally spaced apart a distance A for purposes of simplicity in this example. FIG. 2 illustrates portions of the subject matter of FIG. 1 enlarged for clarity.
With the memory cell array 24 (mounted on a substrate 26) oriented as shown in FIGS. 1A, 2A, the plane of the array 24 is substantially perpendicular to the direction of the flow of neutrons 22 (θ indicates the angle between the plane of the array 24 and the direction of flow of neutrons 22, in this case θ1=90°). In this situation, the spacing of the neutrons 22 impinging on the array 24 is substantially the same as the spacing A. A reading of intensity I of the neutron field 20 taken in accordance with the above procedure will indicate an intensity of, for example, I1. If the memory cell array 24 is oriented in the same neutron field 20 as shown at FIG. 1B, 2B (array 24 rotated counterclockwise relative to FIGS. 1A, 2A), with the plane of the array 24 not substantially perpendicular to the direction of flow of neutrons 22 but at an angle θ2 relative thereto, the spacing B of the neutrons 22 impinging on the array 24 is greater than the spacing A in the previous example. With this being the case, over a given period of time, the array 24 will be exposed to a smaller number of neutrons 22 than in the example of FIGS. 1A, 2A, decreasing the percentage of state changes in the array 24 as compared to the example of FIGS. 1A, 2A. Indeed, it will be seen that, with reference to FIG. 2B, the reading of intensity with the memory cell array 24 oriented as shown at FIGS. 1B, 2B isI=ksin θwhere k is a constant, and θ is the angle between the direction of flow of neutrons 22 and the plane of the array 24.
Likewise, if the memory cell array 24 is oriented as shown at FIGS. 1C, 2C (array 24 rotated clockwise relative to FIGS. 1A, 2A), with the plane of the array 24 not substantially perpendicular to the direction of flow neutrons 22 but at an angle θ3 relative thereto, the spacing C of the neutrons 22 impinging on the array 24 is greater than the spacing A in the example of FIG. 1A, 2A. Which this being the case, over a given period of time, the array 24 will be exposed to a lower number of neutrons 22 than in the example of FIGS. 1A, 2A, decreasing the percentage of state changes in the array 24 as compared to the example of FIGS. 1A, 2A.
Indeed, the above cited formula indicates a maximum intensity reading at θ=90° (sin θ=1, FIG. 1A, 2A), which will readily be seen to be the case in reviewing FIGS. 1 and 2 in their entirety.
Thus, it will be seen that the level of intensity of the neutron field 20 indicated by the present device is dependent on the orientation of the device relative to the direction of flow of the neutrons 22.
In addition, while a level of intensity is read with the army 24 in a variety of positions relative to the direction of flow of neutrons 22, no indication is given as to the direction of neutron flow, i.e., the direction of the source of neutrons relative to the array 24.
Therefore, what is needed is a neutron detecting device which is capable of properly measuring the intensity of a neutron field and indicating the direction of the source of neutrons.